Differentiation with drift: a spatio-temporal genetic analysis of Galapagos mockingbird populations (Mimus spp.)

摘要

Small and isolated island populations provide ideal systems to study the effects of limited populationsize, genetic drift and gene flow on genetic diversity. We assessed genetic diversity within and differ-entiation among 19 mockingbird populations on 15 Galapagos islands, covering all four endemicspecies, using 16 microsatellite loci. We tested for signs of drift and gene flow, and used historicspecimens to assess genetic change over the last century and to estimate effective population sizes.Within-population genetic diversity and effective population sizes varied substantially among islandpopulations and correlated strongly with island size, suggesting that island size serves as a good pre-dictor for effective population size. Genetic differentiation among populations was pronounced andincreased with geographical distance. A century of genetic drift did not change genetic diversity onan archipelago-wide scale, but genetic drift led to loss of genetic diversity in small populations,especially in one of the two remaining populations of the endangered Floreana mockingbird.Unlike in other Galapagos bird species such as the Darwin's finches, gene flow amongmockingbird populations was low. The clear pattern of genetically distinct populations reflects theeffects of genetic drift and suggests that Galapagos mockingbirds are evolving in relative isolation.